U.S. patent number 5,128,911 [Application Number 07/611,799] was granted by the patent office on 1992-07-07 for optical disc reproducing apparatus having an automatic gain control circuit.
This patent grant is currently assigned to Matsushita Electric Industrial Co., Ltd.. Invention is credited to Kenzo Ishibashi, Motoshi Ito, Shunji Ohara.
United States Patent |
5,128,911 |
Ito , et al. |
July 7, 1992 |
Optical disc reproducing apparatus having an automatic gain control
circuit
Abstract
An appparatus for preventing an undesirable increase in the gain
of an automatic gain control circuit (AGC circuit) when an optical
pickup of an optical disc reproducing apparatus performs a track
jump operation. A detected signal of an optical pickup is applied
to an AGC circuit for equalizing the level of the detected signal.
The output of the AGC circuit is detected by an envelope detector
and compared with a reference level by a comparator. The output of
the comparator is applied to the AGC circuit as a gate signal for
holding the gain of the AGC circuit to a predetermined value during
tracing of a nonrecorded part of a track. Additionally, a jump
signal, which is output from a jump signaling device during a track
jump operation of the optical pickup, is applied to the AGC circuit
to hold the gain to a predetermined value during a track jump
operation.
Inventors: |
Ito; Motoshi (Toyonaka,
JP), Ohara; Shunji (Higashiosaka, JP),
Ishibashi; Kenzo (Moriguchi, JP) |
Assignee: |
Matsushita Electric Industrial Co.,
Ltd. (Kadoma, JP)
|
Family
ID: |
17900051 |
Appl.
No.: |
07/611,799 |
Filed: |
November 13, 1990 |
Foreign Application Priority Data
|
|
|
|
|
Nov 20, 1989 [JP] |
|
|
1-301696 |
|
Current U.S.
Class: |
369/53.28;
369/124.11; 369/44.28; 369/44.29; 369/44.35; 369/44.36; G9B/7.045;
G9B/7.091 |
Current CPC
Class: |
G11B
7/08517 (20130101); G11B 7/0941 (20130101) |
Current International
Class: |
G11B
7/085 (20060101); G11B 7/09 (20060101); G11B
007/00 () |
Field of
Search: |
;369/44.28,44.29,44.35,44.36,32,30,33,54,44.25,44.26,116,44.34,44.39,44.32
;360/78.04 ;358/342,907 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Envall, Jr.; Roy N.
Assistant Examiner: Dang; Hung
Attorney, Agent or Firm: Cushman, Darby & Cushman
Claims
What is claimed is:
1. An optical disc reproducing apparatus comprising:
optical signal detecting means for detecting light reflected by an
optical disc,
automatic gain control means having a controllable gain for
equalizing an amplitude of a detected signal of said optical signal
detecting means and for keeping said controllable gain at a first
and second predetermined gain in response to a gate signal and a
jump signal, respectively,
an envelope detector for detecting an output of said automatic gain
control circuit,
a comparator for comparing an output level of said envelope
detector with a predetermined reference level and outputting said
gate signal when said output level is lower than said reference
level, and
jump signal applying means for applying a jump signal to said
automatic gain control means during a track jump operation of said
optical signal detecting means.
2. An optical disc reproducing apparatus comprising:
optical signal detecting means for detecting light reflected by an
optical disc,
automatic gain control means having a controllable gain for
equalizing an amplitude of a detected signal of said optical signal
detecting means and for keeping said controllable gain at a first
and second predetermined gain in response to a gate signal,
an envelope detector for detecting an output of said automatic gain
control circuit,
a comparator for comparing an output level of said envelope
detecting means with a predetermined reference level and outputting
said gate signal when said output level is lower than said
reference level, and
switch means for interrupting communication of said detected signal
to said comparator during a track jump operation, said switch means
disposed at a junction between one of an output of said optical
signal detecting means and an input of said automatic gain control
means, between an output of said automatic gain control means and
an input of said envelope detector, and between an output of said
envelope detector and an input of said comparator.
Description
FIELD OF THE INVENTION AND RELATED ART STATEMENT
1Field of the Invention
The present invention relates generally to an opticaldisc
apparatus, and more particularly to an optical disc reproducing
apparatus having an automatic gain control circuit which is
controlled by a jump signal when crossing from one track to
another.
2. Description of the Related Art
FIG. 1 is the circuit of an optical disc reproducing apparatus in
the prior art. Referring to FIG. 1, optical signal detecting means
comprises an optical pickup 1A having a PIN photodiode 101 for
detecting a reflected light from an optical disc and a
current-voltage convertor 1 for converting the detected current
signal of the PIN photodiode 101 into a voltage signal. The output
of the current-voltage convertor 1 is applied to an amplifier 2.
The output of the amplifier 2 is applied to an AGC circuit 3
(automatic gain control circuit) for keeping an amplitude of the
detected optical signal to a predetermined level.
In the optical disc, information is recorded by a series of pits 5
on a track T1 as shown in FIG. 2(a), and the track comprises a part
having a series of the pits (hereinafter is referred to as recorded
part) and a part having no pit (hereinafter is referred to as
nonrecorded part). When the recorded part of the track T1 is traced
by the optical pickup 1A, the detected signal of the waveform shown
in FIG. 2(b), for example, is output, but when the nonrecorded part
is traced thereby, no detected signal is output. In general, the
amplitude of the detected signal of address pits is different from
that of data pits. Additionally, the amplitude of the detected
signal is varied by the material of the optical disc itself.
Therefore, the amplitude of the detected signal needs to be
equalized by the AGC circuit 3. The AGC circuit 3 is made to have a
sufficiently rapid response speed for following the variation of
the detected signal of the optical pickup 1A. The output of the AGC
circuit 3 is applied to a further amplifier (not shown) for
reproducing the information of the optical disc via a terminal 7.
The output of the AGC circuit 3 is also applied to an input signal
detecting circuit 4 shown in FIG. 1.
The input signal detecting circuit 4 is comprised of an envelope
detector 4A having a diode 401, a resistor 402 and a capacitor 403,
and a comparator 404. The output of the AGC circuit 3 is input to
the envelope detector 4A. The detected signal of the envelope
detector 4A is applied to the invert input (-) of the comparator
404, and a reference voltage RV is applied to the noninvert input
(+) thereof. The output of the envelope detector 4A is compared
with the reference voltage RV in the comparator 404. When the
output of the envelope detector 4A is lower than the reference
voltage RV, a gate signal 8 is output from the comparator 404. The
output of the AGC circuit 3 is normally lower than the reference
voltage RV when the optical pickup 1A traces the nonrecorded part
of a track. Thus the gate signal 8 represents tracing of the
nonrecorded part of the optical disc. The gate signal 8 is applied
to the AGC circuit 3, and thereby the gain of the AGC circuit 3 is
held to a predetermined value which is comparatively lower. The
response speed of gain control of the AGC circuit 3 is less than
the variation speed of a detected signal of the optical pickup 1A
when tracing the boundary of the recorded part and nonrecorded part
on the same track. Consequently, when the optical pickup 1A traces
from the recorded part to the nonrecorded part on the same track,
an increase in the gain of the AGC circuit 3 cannot overtake a
decrease in the level of the detected signal. The output of the AGC
circuit 3 decreases in compliance with the input signal thereof
which corresponds to the detected signal of the optical pickup 1A.
Thus, the invert input level of the comparator 404 is lowered, and
the gate signal is output from the comparator 404. The gain of the
AGC circuit 3 is held to the predetermined value by the gate
signal.
In general, the optical disc has a spiral track or a plurality of
concentric tracks. In reproduction of the recorded signal of the
above-mentioned tracks, the optical pickup 1A in which the PIN
photodiode 101 is mounted is shifted in the radial direction of the
optical disc. And when the optical pickup 1A is shifted from a
track to other track for reproducing the data of other addresses,
the optical pickup 1A crosses from one track to another track.
Referring to FIG. 2(a) two-dotted lines A1 represents a recorded
track T1 having the series of the pits 5, and two-dotted lines A2
represents a nonrecorded track T2 neighboring to the track T1. The
path 6 of the optical pickup 1A in the shift operation is
represented by a solid line. Note the solid line is not
perpendicular from one track to the other because the optical disc
is rotated.
FIG. 2(b) is a waveform of the detected signal of the optical
pickup 1A in the shift operation along the path 6. Since the
diameter of the detected area of the optical pickup 1A is larger
than the diameter of the pit 5, the amplitude of the detected
signal gradually decreases in accordance with the shift of the
optical pickup 1A from track T1 to track T2 as shown in FIG.
2(b).
The detected signal is amplified by the amplifier 2 and is applied
to the AGC circuit 3. Since the response speed of the AGC circuit 3
is sufficiently rapid with respect to a varying speed of the
detected signal in the shift operation, the AGC circuit 3 varies
the gain in compliance with the variation of the detected signal.
Consequently, when the optical pickup 1A moves to nonrecorded track
T2, the gain of the AGC circuit 3 increases, and a noise component
of the detected signal of the optical pickup 1A is amplified and
output from the AGC circuit 3. The noise component detected by the
envelope detector 4A is applied to the comparator 404. Since the
gain of the AGC circuit 3 is generally made considerably higher,
the level of the noise becomes higher than the reference voltage
RV, and thus the gate signal is not output for limiting the
increase of the gain of the AGC circuit 3. Such amplification of
the noise component is not desirable for reproduction of the
optical disc.
OBJECT AND SUMMARY OF THE INVENTION
An object of the present invention is to provide an optical disc
reproducing apparatus having an automatic gain control circuit for
preventing undesirable increase of the gain thereof when an optical
pickup of the optical disc reproducing apparatus cross from one
track to another track on an optical disc.
The optical disc reproducing apparatus in accordance with the
present invention comprises:
optical signal detecting means for detecting light reflected by an
optical disc,
automatic gain control means for controlling a gain to equalize an
amplitude of the detected signal of the optical signal detecting
means and for keeping the gain to a predetermined value when a gate
signal is applied thereto.
an envelope detector for detecting the output of the automatic gain
control circuit,
a comparator for comparing the output level of the envelope
detector with a predetermined reference level and outputting the
gate signal when the output level of the automatic gain control
means is lower than the reference level, and
jump signal applying means for applying a jump signal to the
automatic gain control means to hold a predetermined gain of the
automatic gain control means during a track jump operation of the
optical signal detecting means.
According to the present invention, when the optical pickup travels
from a recorded track to a non-recorded track, an abnormal increase
in the gain of the automatic gain control means by reduction of a
detected signal level of the optical pickup is prevented and can be
maintained to an adequate gain.
While the novel features of the invention are set forth
particularly in the appended claims, the invention, both as to
organization and content, will be better understood and
appreciated, along with other objects and features thereof, from
the following detailed description taken in conjunction with the
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is the circuit of the relevant parts of the optical disc
reproducing apparatus in the prior art;
FIG. 2(a) is an enlarged plan view of the tracks of an optical
disc;
FIG. 2(b) is a waveform of the detected signal of the optical
pickup;
FIG. 3 is a circuit of the relevant parts of an optical disc
reproducing apparatus of a first embodiment in accordance with the
present invention;
FIGS. 4(a), 4(b), 4(c), 4(d), 4(e) and 4(f) are waveforms in
operation of the first embodiment;
FIG. 5 is a circuit of the relevant parts of an optical disc
reproducing apparatus of a second embodiment in accordance with the
present invention;
FIGS. 6(a), 6(b), 6(c), 6(d) and 6(e) are waveforms in operation of
the second embodiment.
It will be recognized that some or all of the Figures are schematic
representations for purposes of illustration and do not necessarily
depict the actual relative sizes or locations of the elements
shown.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 3 is a circuit of a first embodiment of the optical disc
reproducing apparatus in accordance with the present invention.
Referring to FIG. 3, an optical signal detecting means for
detecting a light reflected by the surface of an optical disc
comprises an optical pickup 1A and a current-voltage convertor 1.
The optical pickup 1A comprises a PIN photodiode 101 for detecting
the light reflected by the surface of the optical disc and a
driving mechanism for shifting the optical pickup 101; these are
familiar to one skilled in the art (not shown). The output current
of the optical pickup 1A is converted to voltage signal by the
current-voltage convertor 1. The output of the current-voltage
convertor 1 is applied to an amplifier 2, and is amplified thereby.
The output of the amplifier 2 is applied to an AGC circuit 3 the
gain of which varies inversely proportional to the level of the
input signal. The output of the AGC circuit 3 is input to an input
signal detecting circuit 4.
The input signal detecting circuit 4 comprises an envelope detector
4A composed of a diode 401, a resistor 402 and a capacitor 403 and
an comparator 404. The output of the envelope detector 4A is
applied to the invert input (-) of the comparator 404, and a
reference voltage RV is applied to the noninvert input (+) thereof.
The amplified signal of the AGC circuit 3 is output to a terminal 7
to be supplied to other circuits. The output of the comparator 404
(hereinafter referred to as gate signal) is applied to an input of
an OR gate 9. A jump signal for instructing track jump of the
optical pickup 1A is applied to the other input 10 of the OR gate
9. The output of the OR gate 9 (hereinafter referred to as gain
hold signal) is applied to the AGC circuit 3.
Operation of the first embodiment is elucidated hereafter. The
detected current signal of the optical pickup 1A is converted to a
voltage signal by the current-voltage convertor 1 and is amplified
by the amplifier 2, and is applied to the AGC circuit 3. The AGC
circuit 3 has responsibility to equalize the level of the detected
signal which varies due to inequality of the surface of the optical
disc. The output of the AGC circuit 3 is detected by the envelope
detector 4A and is applied to the invert input (-) of the
comparator 404. In the comparator 404, when the invert input level
is higher than the reference voltage RV, which is applied to the
noninvert input (+), there is no output at the output terminal 8 of
the input signal detecting circuit 4.
When the optical pickup 1A traces on the nonrecorded part of a
track, the detected signal is not output, and thus no input signal
is applied to the AGC circuit 3. Consequently, the output level of
the AGC circuit 3 is in a low level, and the invert input level of
the comparator 404 is also in the low level. When the invert input
level is lower than the reference voltage RV, the comparator 404
outputs the gate signal for representing tracing of the nonrecorded
part of the track at the output terminal 8. The gate signal is
applied to the AGC circuit 3 through the OR gate 9. The AGC circuit
3 is controlled by the gate signal to hold the gain thereof to a
predetermined value.
FIGS. 4(a) through 4(f) are waveforms of signals in operation of
the first embodiment, and the abscissa is graduated by a time which
corresponds to a position on a track of the optical disc.
FIG. 4 (a) is a waveform of the input signal of the AGC circuit 3.
From a time T0 to a time T1, a recorded part of a track is detected
by the optical pickup 1A, and the detected signal shown by FIG.
4(a) is applied to the AGC circuit 3. At the time T1, a nonrecorded
part of the track is detected by the optical pickup 1A, and the
input of the AGC circuit 3 disappears. At a time T2, a recorded
part of the track is detected again. The output of the AGC circuit
3 is shown in FIG. 4(b). Referring to FIG. 4(b), variation of the
amplitude in the input signal is considerably equalized. The output
of the AGC circuit 3 is detected by the envelope detector 4A. The
detected signal shown in FIG. 4(c) is applied to the invert input
of the comparator 404. The invert input level of the comparator 404
decreases at the time T1 as shown in FIG. 4(c) and becomes lower
than the reference voltage RV. Consequently, the comparator 404
outputs the gate signal for representing tracing of nonrecorded
part as shown in FIG. 4(d). The gate signal is applied to the OR
gate 9 as shown in FIG. 3, and the gain hold signal shown in FIG.
4(f) is applied to the AGC circuit 3 between the time T1 and the
time T2, and thereby the gain thereof is maintained to the value
which is identical with the gain before the gain hold signal was
applied between the time T1 and the time T2.
At a time T3, as shown in FIG. 4(e), a jump signal for instructing
jump of track is applied to the optical pickup 1A (not shown). The
optical pickup 1A is moved to a radial direction of the optical
disc and is shifted to a neighboring track in compliance with the
jump signal. Since the optical pickup 1A goes away from the track,
the amplitude of the detected signal of the optical pickup 1A
(input of AGC circuit 3) gradually decreases as shown in FIG. 4(a).
On the other hand, the jump signal is also applied to the OR gate 9
at the time T3, and the gain hold signal is applied to the AGC
circuit 3. Thus the gain of the AGC circuit 3 is held to the
predetermined value. Since the level of the invert input of the
comparator 404 becomes lower than the reference voltage RV at a
time T4 by reduction of the output level of the AGC circuit 3, the
gate signal is output from the comparator 404 and is applied to the
OR gate 9. Thus the gain hold signal is applied to the AGC circuit
3. The jump signal disappears at a time T5 as shown in FIG. 4(e),
and the track jump operation is completed. Since the track on which
the optical pickup 1A is shifted is a nonrecorded track, the output
level of the AGC circuit 3 is substantially zero, and the gate
signal is continuously output.
The response of the AGC circuit 3 speed of gain control is made
less than the variation speed of the detected signal, and thus the
control of gain can not follow the signal level variations when
crossing over the boundary of the recorded part and the nonrecorded
parts of the track. However, when the optical pickup jumps from the
track having the recorded part to a track having no recorded part,
a detected signal of the recorded part gradually decreases with the
movement of the optical pickup 1A in the radial direction of the
optical disc. The gain of the AGC circuit 3 increases in inversely
proportional to the increase of the detected signal. In the first
embodiment, the increase of the gain is prevented by the jump
signal.
FIG. 5 is a circuit of a second embodiment in accordance with the
present invention. In the second embodiment, an input cutoff
circuit 12 is provided between the amplifier 2 and the AGC circuit
3. The input cutoff circuit 12 comprises a moving contact 12A and
transfer contacts 12B and 12C. The moving contact 12A is activated
by the jump signal applied to input 10. The output of the
comparator 404 is directly input to the AGC circuit 3. The
remaining components functioning in the same manner as in the
arrangement of FIG. 3 are designated by the like numerals as used
with corresponding parts shown in FIG. 3.
In the second embodiment, the output of the amplifier 2 is
communicated to the input of the AGC circuit 3 through the contact
12B and the moving contact 12A during tracing of a track of the
optical disc. In the track jump operation of the optical pickup 1A,
the jump signal is applied to the input cutoff circuit 12, and the
moving contact 12A is contacted to the contact 12C, and the input
of the AGC circuit 3 is grounded. Consequently, the invert input of
the comparator 404 becomes zero, and the gate signal is output from
the comparator 404. Thus, the gain of the AGC circuit 3 is
determined by the gate signal.
Though the input cutoff circuit 12 is represented by a symbol of
mechanical switch in FIG. 5 it is also possible to use a CMOS
analogue switch (for example, Model MM54HC4066 or MM74HC4066 in the
INDUSTRIAL LINEAR IC MANUAL ('89 Part 1) of the National
Semiconductor Corp.).
The above-mentioned operation is shown in FIGS. 6(a) through 6(e).
The optical pickup 1A traces the recorded part of a track between
the times T0 and T1 and between the times T2 and T3. A nonrecorded
part of the track is traced between the times T1 and T2. Operation
of the optical disc reproducing apparatus between the times T0 and
T3 is identical with that of the first embodiment. Track jump
operation is started at time T3 and the input cutoff circuit 12 is
activated by the jump signal as shown in FIG. 6(e). Consequently,
the output of the AGC circuit 3 disappears as shown in FIG. 6(b)
because the input of the AGC circuit 3 is grounded by the input
cutoff circuit 12. Thus the comparator 404 outputs the gate signal
as shown in FIG. 6(d), and the gain of the AGC circuit 3 is held to
a predetermined value. In the second embodiment, the input cutoff
circuit 12 can be inserted between the optical pickup and the
current-voltage convertor 1, between the current-voltage convertor
1 and amplifier 2, between the AGC circuit 3 and the input signal
detecting circuit 4 or between the envelope detector 4A and
comparator 404.
Although the invention has been described in its preferred form
with a certain degree of particularity, it is understood that the
present disclosure of the preferred form may be changed in the
details of construction and the combination and arrangement of
parts without departing from the spirit and the scope of the
invention as hereinafter claimed.
* * * * *